The present invention relates to a device, having at least one micromechanical surface structure patterned on a silicon substrate and one protective cap covering the at least one surface structure, as well as a method for manufacturing the device.
Previous micromechanical devices. These include a silicon substrate, on whose surface a polycrystalline silicon layer is epitaxially grown using known processes. Micromechanical structures, e.g. seismic masses of sensor elements, micromotor actuators, or other movable structures, are produced in this silicon layer. For example, the patterning is attained by defined etch attacks from the upper side of the polycrystalline silicon, flexibly suspended structures being attainable by area-specific undercutting.
In order to protect the micromechanical structures from outside influences during normal use of the devices, these are known to be provided with a covering protective cap. In this case, this protective cap is known to be manufactured, in accordance with the device to be covered, as a patterned silicon wafer with which the wafer having the surface structure is joined. In order to form this joint, the cap wafer is provided with a meltable glass by means of screen printing. The cap wafer is subsequently aligned to the base wafer, and they are joined under pressure and at a temperature of approximately 400xc2x0 C.
In this case, it is disadvantageous that the devices can only be manufactured by means of a relatively expensive manufacturing process using screen-printed, meltable glass. A particular disadvantage is, that in the joining procedure subsequent to the screen printing of the meltable glass, a certain amount of the meltable glass is unavoidably pressed out of the joining location or locations between the cap wafer and base wafer. In order to prevent the micromechanical structures from being influenced by the emerging glass, a relatively large contact or bonding surface is needed between the cap wafer and the base wafer. For example, if a bonding area is printed over with an approximately 500 xcexcm wide glass layer, this results in an actual requirement of approximately 700 xcexcm in the subsequent joining procedure, due to the glass spreading out laterally. This additionally required surface is not available for arranging functional structures of the device, so that the size of the known constructed devices are correspondingly large.
An additional disadvantage of the known devices is that a great deal of effort is required to hermetically seal them, since connecting the cap wafer using meltable glass applied by screen printing technically only allows a partial vacuum.
A further disadvantage is, that after joining the cap wafer to the base wafer, it is only possible to test the newly encapsulated micromechanical surface structures by measuring. Optical testing is not possible.
The device according to the present invention offers the advantage of being able to be manufactured using simple and reliably controllable method steps. Because the cap is formed from a glass wafer, the covering glass wafer can be joined to the base wafer of the device using robust methods suitable for mass production. In particular, when the surface of the base wafer facing the glass wafer is formed with a defined residual roughness, especially  less than 40 nm, the glass wafer can be directly applied to the base wafer without applying intermediate adhesion-promoting layers.
Surprisingly, it was found that residual roughnesses  less than 40 nm can be reproduced using, for example, so-called CMP methods (chemical mechanical polishing) for polycrystalline silicon layers, in which the micromechanical surface structures are laid out. Because the upper side pointing towards the glass wafer can be planarized in such a high-quality manner, joining techniques can be used that supercede the step, having the above-mentioned disadvantages, of inserting an additional adhesive agent, especially meltable glass applied by screen printing.
It is especially preferable to join the glass wafer to the base wafer by anodic bonding. By this means, relatively small bonding surfaces can be attained, which require a correspondingly reduced amount of space on the device. Therefore, the bonding surfaces can be placed closer to the functional structures of the device, so that their total surface requirement is reduced.
A further preferred refinement of the present invention provides for the glass wafer being optically transparent. This enables the micromechanical surface structures encapsulated by the glass wafer to undergo an optical examination after the device has been manufactured. It is also of great advantage, that the movements of the micromechanical structures can be evaluated optically, in that the micromechanical structures have, for example, active and/or passive optical elements with which optical signals passing through the transparent glass wafer can be evaluated.
It is also preferable, that the present device according to the invention can especially enclose vacuums of up to 1 mbar. In this manner, the micromechanical structures can be used very advantageously as seismic masses of rate-of-rotation sensors, in which a high-quality vacuum is used to attain a sufficient vibrational quality.
A further preferred refinement of the present invention provides at least one electrode arranged on the side of the glass wafer facing the micromechanical structures. Apart from covering the micromechanical structures, this allows the glass wafer to be simultaneously used for detecting possible deflections of the micromechanical structures, in that the electrode is, e.g. a component part of a capacitive evaluating arrangement, which detects changes in distance between the glass-wafer electrode and at least one micromechanical structure.